A video posted on the Internet showed the chopper engulfed in flames and spinning out of control shortly before it hit the ground amid bursts of gunfire near a mosque. Rebels shout “Allahu Akbar!” or God is great, as the helicopter goes down. The authenticity of the video could not be independently verified.

Syria's lightly armed rebels have grown bolder and their tactics more sophisticated in recent months. There have been claims of fighters shooting down helicopter gunships in the past, though the government has never confirmed it.

With its forces stretched thin by fighting on multiple fronts, Assad's regime has been increasingly using air power against the rebels — both helicopters and fighter jets.

The Britain-based Syrian Observatory for Human Rights, which along with other activists reported the chopper crash, said there was intense fighting between troops backed by helicopter gunships and rebels in the western Damascus neighbourhood of Jobar. State-run media confirmed the crash in the district of al-Qaboun, which is near Jobar and a hotbed of Sunni Muslim rebels fighting to topple President Bashar Assad. But the reports gave no details about the cause of the crash

The rebels are not known to have any answer to the regime's warplanes except anti-aircraft guns that they mostly use as an anti-personnel weapon. Last month, rebels claimed to have shot down a Russian-made MiG fighter, but the government blamed the crash on a technical malfunction.

The military has for more than a month been fighting major battles against rebels in Damascus and its suburbs while engaged in what appears to be a stalemated fight in the north against rebels for control of Aleppo, the nation's largest city and commercial capital. The government has recently intensified its offensive to recapture districts in Damascus and its suburbs that have fallen into rebel hands.

Over the weekend, evidence mounted of mass killings by government forces in the Damascus suburb of Daraya.

Activists reported that regime forces went on a days-long killing spree after they seized Daraya from rebels Thursday. Reports of the death toll ranged from more than 300 to as many as 600. It was impossible to independently verify the numbers because of severe restrictions on media coverage of the conflict.

Video posted on the Internet by activists showed rows of bodies, many of them men with gunshot wounds to their heads. During mass burials on Sunday, bodies were sprayed with water from hoses — a substitute for the ritual washing prescribed by Islam in the face of so many dead. The gruesome images appeared to expose the lengths to which Assad's authoritarian regime was willing to go to put down the rebellion that broke out in March last year.

In the north, thousands of Syrians fleeing violence were stuck at the border with Turkey after Turkish authorities blocked access to any more refugees while they rushed to build more camps to accommodate the deluge.

The refugee crisis is just one of many examples of how the civil war is spilling across borders into neighbouring countries such as Turkey, Lebanon and Jordan — all of which have seen a massive influx of Syrians.

A Turkish official said the refugees still stuck on the Syrian side will be allowed in “within a day or two” when a new camp near the border is ready. Another official said Turkey was also carrying out more stringent security checks on the refugees, adding to the delays in crossing.

Turkey fears that Kurdish rebels fighting for self-rule in southeast Turkey may be coming in through Syria. There are also concerns that foreign jihadists are moving in and out of Turkey to fight the Syrian regime. The officials spoke on condition of anonymity because of government rules that bar civil servants from speaking to the media without prior authorization.

The official Syrian News Agency, SANA, also reported that authorities released 378 people detained for participation in peaceful street protests. It said those freed were never involved in acts of violence, an indirect admission that scores of people were detained for simply taking part in peaceful anti-government protests. The prisoners were released at the police headquarters in Damascus and the central city of Homs.

Authorities have issued similar pardons in the past, a practice apparently designed to isolate the rebels and create the image of a compassionate regime.

Vice-President Farouk al-Sharaa said Syria approved of an initiative Iran plans to present at a world gathering of nonaligned nations that will be held in Tehran later this week. Iran says it plans talks on a peace plan to end Syria's civil war, but hasn't provided any details.

Any Iranian initiative proposing dialogue between the government and the opposition would be a nonstarter because rebels refuse to talk to Assad and because of Tehran's close bonds with his regime.

Al-Sharaa, in a statement issued by his office, said the way to settle the conflict without preconditions is by “achieving a ceasefire by all parties and launching a national dialogue.”